The present invention relates to a microwave equaliser with internal amplitude correction, preferably for application in satellite communications. More specifically, the invention relates to a microwave equaliser comprising resonant cavities, operating in a reflexive configuration, and having tuning means to achieve equalising of the distortions that can arise during the transmission of a signal, in both group delay and in amplitude. The present invention is particularly useful for applications which require very precise, high quality equalisation, in both group delay and in amplitude, as for example in the case of channel filters normally employed in satellite communications payloads.
In practice, conventional equalisers designed to perform both delay and amplitude equalisation have the drawback that both the equaliser itself and the device for coupling the latter to a main filter, for example a circulator, behave in a non-ideal manner, for which reason simultaneous equalisation of the two parameters cannot be achieved; or at best the simultaneous equalisation so obtained is of a quality and accuracy significantly less than that desired.
A conventional type solution to this problem is to employ additional equalisation means corresponding to each type of equalisation that permits an independent control for group delay and for amplitude, respectively. In this sense, in current practice, the choice has been to add an independent amplitude equaliser to the delay equaliser. In certain cases, this solution can consist in the use of a variable, resistive means the setting of which affects the amplitude of the signal to be equalised. However, this solution implies, in turn, a significant economic impact as well as additional weight to be borne due to the mass of said additional equaliser. This effect is a drawback of considerable magnitude, particularly in the case of satellite communications.
The use of a variable, resistive means to perform delay and amplitude equalisation is known through the patent U.S. Pat. No. 4,524,337, incorporated in the present specification by reference. This document proposes an equaliser which comprises a 90xc2x0 hybrid matrix formed by a toroidal transformer, a reactive network to shift the phase of a signal applied to said matrix that permits group delay correction and consists of a pair of capacitors each of which being connected to a variable inductor; and an RC network to provide amplitude correction to the signal at the matrix output. The RC network, in turn, comprises three resistors, one of which is a variable resistor connected across two connection points of the transformer such that one connection point constitutes the input and the other connection point is the equaliser output. The setting of said variable resistor permits the amplitude response to be varied. In this way, it is intended to achieve a correction in the amplitude that is essentially independent of the delay adjustment, which is achieved via the pertinent delay correcting circuitry.
As may be appreciated, the equaliser described in patent U.S. Pat. No. 4,524,337 requires a relatively high number of components, i.e. a toroidal transformer, several capacitors, resistors and inductors that inevitably give rise to an increase in the cost and complexity of the circuit.
Moreover, an equaliser of this type is not generally suitable for equalising high frequency signals, as is the case of the equaliser of the present invention.
As a result, there exists a need for a high frequency equaliser capable of correcting group delay and amplitude independently of each other, i.e. the correction of one type of distortion does not negatively affect the correction of the other distortion, such that the economic cost and the complexity of the circuitry required are reduced substantially.
To overcome the problems outlined above, the equaliser with internal amplitude correction object of the present invention is proposed, which comprises resonant cavities of conventional type and works in a reflexive configuration. In said equaliser use is made of a variable, resistive tuning means, preferably a resistive screw, which in a coupling arrangement with an input signal injection means that projects into a cavity of the equaliser, produces the effect of selectively introducing losses in certain segments of the frequency band, said effect being variable both in magnitude and in the segment of the band affected, depending on the position of said resistive screw with respect to said input signal injection means.
More specifically, the equaliser of the present invention is characterised in that it comprises at least one input signal injection means that projects into said resonant cavity for injecting signal into said resonant cavity, and at least a first resistive tuning means for absorbing electromagnetic energy through coupling with respect to said input signal injection means, in such a manner that at least one relative position between said first resistive tuning means and said input signal injection means is variable for producing selectively an amplitude attenuation effect in said signal.
According to a preferred embodiment of the invention, at least a first resistive tuning means effects a first change of position in the direction of a plane substantially perpendicular to an input signal injection means in order to produce a variation in one parameter of a frequency response of the equaliser.
According to other preferred embodiment of the invention, at least a first resistive tuning means effects a second change of position in the direction of a plane substantially parallel to an input signal injection means in order to determine the degree of symmetry between respective ends of the response in the equaliser frequency band.
According to another preferred embodiment of the invention, the equaliser comprises at least a first resistive tuning means that is mounted adjacent to at least a third tuning means, each being displaceable in independent form and capable of producing said first and/or said second change of position in a selective manner.
According to another preferred embodiment of the invention, the equaliser comprises also a second resistive tuning means housed in a cavity of the equaliser for producing losses in a frequency band of the signal in a symmetrical manner.
According to another preferred embodiment of the invention, the first and second resistive tuning means are screws that include material capable of absorbing electromagnetic energy.
According to another preferred embodiment of the invention, the input signal injection means is a signal input probe.
In addition, a further object of the invention is to provide a microwave filter incorporating the equaliser described above.
Another object of the invention is to provide a microwave filter that employs, additionally, the second resistive tuning means described above.
These and other advantageous features of the invention can be understood in greater detail in the embodiment examples that are described below with the assistance of the attached figures.